1. Field of the Invention
This invention relates to a novel method for developing positive photoresists, and more particularly to a process for developing alkali soluble resin/diazoketone photoresists so as to increase the developing rate difference, termed contrast, between the exposed and unexposed resist. The invention resides in the discovery that when a photoresist film is contacted with a aqueous solution of an organic base and a cationic surfactant prior to development in a aqueous solution of an organic base and a fluorochemical surfactant, the developing rate of the unexposed resist is virtually zero resulting in very high contrast. Futhermore the exposed photoresist develops cleanly to the substrate and the patterns have vertical edge profiles.
2. Description of the Prior Art
Photoresists are typically films which change their solubility response to a developer solution after exposure to an irradiation source, such as ultraviolet light. As a consequence of the exposure, a different dissolution rate in the developer exists between the exposed and unexposed (masked over) portions of the photoresist film thus producing the mask pattern in the photoresist film after development. Those photoresists which become more soluble in the exposed regions are referred to as positive photoresists and are the type addressed in this invention.
Positive photoresists typically consist of an aqueous alkaline soluble resin, such as novolac resin or poly-(p-hydroxystyrene), and a diazonaphthoquinone sulfonic acid ester dissolved in an organic solvent. The resist is usually applied by a spin casting technique to silicon wafers which may have a thin coating of silicon dioxide, aluminum, silicon nitride, glass or other material typically used in the fabrication of integrated circuits.
The coated wafers are exposed to light through a mask which provides a pattern for building the various circuit components on the wafer. The pattern is developed by dipping, spraying or placing a puddle of developer solution on the wafer.
The developer used is usually an aqueous base solution of either inorganic bases such as KOH, NaOH and Na.sub.2 SiO.sub.3 or organic bases such as tetramethylammonium hydroxide and (2-hydroxyethyl)trimethylammonium hydroxide. The inorganic, metal ion containing bases are not preferred for integrated circuit patterning due to possible mobile ion contamination of the devices.
Typically, the alteration of the solubility of the photoresist in the developer is only a relative change; consequently, even the unexposed portions of the photoresist dissolve to some extent. Any process that enhances the developing rate difference between the relatively soluble exposed and relatively insoluble unexposed photoresist film is advantageous. It is highly desirable that during development there be as little attack as possible on the unexposed resist under conditions which lead to complete removal of exposed resist down to the substrate. The lower the dissolution rate of the unexposed resist relative to the exposed resist, the higher the contrast. High contrast gives a pattern with well defined vertical side walls and little change in the pattern over irregular surface topography.
Fluorocarbon and organic acid surfactants have been used with inorganic developers to achieve high contrast as disclosed in U.S. patent application Nos. 06/505,571 and 06/660,600, respectively. Likewise, quaternary ammonium surfactants have been used with organic base developers. However, in both cases the surfactant is quickly depleted, typically after just a single dip in the developer bath, thus losing high contrast imparted by the surfactant and making the process difficult to control. In addition, the organic base developer with the quanternary ammonium surfactant does not clean the residue from the exposed areas reliably.
Another developing method, disclosed in U.S. Pat. No. 3,961,100, uses a double develop process without surfactants to improve the sensitivity and reduce the film loss resulting in higher contrast. This represents only a marginal improvement compared to the contrast achieved with this invention.
Another method, disclosed in U.S. Pat. No. 4,212,935 employes an organic solvent soak to reduce the attack of the developer on the unexposed resist film. This treatment removes the lower molecular weight resin from the film and thus lowers the developing rate of the surface of resist. In the exposed areas the developing rate of the surface is still sufficient for the developer to break through to the underlying untreated resist. The result of such treatment is an undercut profile and little attack on the unexposed resist. The process is lengthy and difficult to control as disclosed in IBM Technical Bulletin, Vol. 27, No. 1A, pp. 377-378 (June, 1984). The solvents, particularly chlorobenzene, are hazardous to handle.
Other techniques have been disclosed for chemically treating the coated photoresist for generating the undercut profile for metal lift-off processes. Various such techniques are disclosed in U.S. Pat. No. 4,212,935, but do not address the method and compositions disclosed in this invention in order to achieve high contrast, no unexposed film loss and the other benefits of the process to be disclosed in this invention.
A need exists for an organic base, aqueous developer which is capable of providing a high contrast value (gamma) of at least five (5) without sacrificing sensitivity and which attains the foregoing advantages and minimizes the stated drawbacks.